People who wear pajamas to Walmart. I should know, I'm one of them.
Goths used to scare me before I found out they're actually people too and I just don't understand Emo folks. Or Hipsters.
Also, I don't mind Norwegian Black Metal (Mr. Cat is correct in that they are crazy, however) but I am distressed and slightly confused by those folks who follow the "band" ICP and call themselves Juggalos.
Truthfully, though, the one subculture that boggles my mind the most isn't really defined as a subculture, but I think it should be. Those are the folks who want to be just like everybody else. They fix their hair the same way, wear the same clothes, like the same movies and music - even if they don't like the same movies and music, they will pretend they do just to be accepted by the folks they're trying to be exactly like. Then they grow up and all drive the same kind of car - how do they ever find the right one in the parking lot at Ikea?